Bibliography: p. 173-185.Approximately 750 square kilometers of the Southeastern Cariboo Mountains near Thunder River were mapped in detail. The Semipelite-Amphibolite and Middle Marble divisions of the Horsethief Creek Group crop out in that region. They are overlain by the Upper elastic Division of the Horsethief Creek Group which grades upwards into Kaza Group lithologies. The Semipelite-Amphibolite Division is in excess of 500 m thick, with its base not exposed within the study area. The Middle Marble Division varies greatly in thickness, with a maximum thickness of 300 m. Approximately 3800 m of Kaza Group section were measured. The Kaza Group is overlain by slates of the Isaac Formation. Earlier workers correlated the Kaza Group with the basal division of the Horsethief Creek Group and with the Middle Miette Group. In light of the findings in the Thunder River area these correlations cannot all be correct. The Horsethief Creek Group underlies the grits of the Kaza Group. The Kaza Group is still considered correlative with the Middle Miette Group. Shales and carbonates overlie both the Middle Miette and Kaza groups. The strata in the Southeastern Cariboo Mountains were subjected to two major phases of folding during the Jura-Cretaceous Columbian Orogeny. The earliest phase resulted in west facing nappe structures. Fl folds are best developed at deep structural and stratigraphic levels. In the highest stratigraphic units studied there are no Fl folds or Sl schistosity developed. F2 folds are northwest plunging, upright to overturned and characterized by a crenulation cleavage. The F2 folds are conical but can only be recognized as such when mapped for large distances parallel to their trace. Metamorphism in the area ranges from sub-biotite to sillimanite grade. The metamorphic peak is synkinematic with D2. At the sillimanite isograd, sillimanite, kyanite and staurolite all coexist. Pressures and temperatures of 4.7 kb and 530 C were estimated for samples from the sillimanite isograd using garnet-biotite geothermometery and garnet-plagioclase geobarometry. The west-side-down North Thompson River Fault, separates the Cariboo Mountains from the Monashees to the east. Pressures and temperatures of 6.0kb and 655 C were estimated for samples from the sillimanite isograd in the Monashee Mountains. This pressure difference (in excess of 1kb) suggests that there is approximately 4 km of vertical displacement on the North Thompson River Fault
